Introduction
[0001] The present invention relates to cellulosic fibrous material comprising a radiation
activatable resin, structures comprising such fibrous material, and absorbent articles
especially disposable absorbent articles, comprising such fibrous materials or structures.
It further relates to a process to make such fibrous material, structures or articles.
Background art
[0002] Cross-linked cellulose for use in absorbent articles is well known, and disclosed
such as in EP-A-0.427.316 (Herron), US-A-5.549.791 (Herron), WO 98/27262 (Westland),
or US-6.184.271 (Westland). Whilst such fibers exhibit useful properties and have
found broad commercial applications, there remains the need for improving such fibers
especially with regard to allowing better balancing of brittleness and resiliency
properties of such fibers. Whilst stiffness is often desired for allowing to maintain
an open structure such as for improved liquid handling, it is with current materials
often linked to increased brittleness of the fibers, creating, for example, undesired
break up during the transport of the fibers from the fiber making and fiber treatment
plant to the fiber user.
[0003] In order to improve on these problems, the present invention relates to the application
of radiation activatable resins to the cellulose fibers and - upon application of
the radiation - to fiber comprising cross-linked radiation activatable resin.
[0004] Radiation curable resins as such are known in the art such as have been disclosed
in DE-38 36 370 (Hintze; BASF), or US-A-5.026.806 (Rehmer; BASF), wherein UV cross-linkable
materials based on (meth-) acrylic acid ester or co-polymers thereof are described
in particular for being used in hotmelt (contact) adhesives and sealing compounds.
Application of photo-curable resins to optical fibers has been disclosed e.g. in WO
99/30843, and the application to non-woven webs is described in US-A-4.748.044. Further,
photocurable, cellulose based compositions are known, which are derived from cellulose
based materials, such as described in JP2298501 (Shin Etsu), or JP-08006252 (Sony),
the latter relating to a general-purpose photosensitive resin composition. In US-A-6.090.236
(Nohr), a process is described to create coatings for a web by radiation induced polymerization
of monomeric or oligimeric materials.
[0005] However, so far it has not been contemplated to exploit radiation induced cross-linking
of polymeric material in the context of cellulosic fibers.
[0006] Henceforth, the present invention aims at providing cellulose based fibers comprising
a radiation activatable cross-linking or curing resin, at structures and especially
absorbent articles comprising such fibers, as well as at the methods of making such
fibers or structures.
[0007] In a particular embodiment, the present invention provides an improved process for
handling cellulosic fiber material, especially when this fiber material is being transported
or stored during the overall handling, with improved fiber properties resulting from
such handling as compared to conventional transporting or storage.
Summary
[0008] The present invention relates to fibrous material comprising cellulosic fibers, whereby
the fibers comprise a polymeric resin with covalently bonded radiation reactive groups,
which are capable of forming cross-linking bonds upon being impacted by radiation
energy. The cellulose based fibers can be crimped, curled, and are preferably flash-dried
fibers. Preferably, the polymeric resin has a T
g of more than 30°C, preferably 50°C, when cross-linked to a degree of cross-linking
of at least 85%, and the radiation activatable groups are selected from the group
consisting of benzophenone, anthraquinone, benzile, xanthones, preferably from the
group of benzophenones. Preferably, the polymeric resin has a polymeric backbone monomer
has molecules selected from the group of ethylene; propylene; vinyl chloride; isobutylene;
styrene; isoprene; acrylonitrile; acrylic acid; methacylic acid; ethyl acrylate; methylmethacrylate;
vinyl acrylate; allyl methacrylate; tripropylene glycol diacrylate; trimethylol propane
ethoxylateacrylate; epoxy acrylates; polyester acrylates; and urethane acrylates.
[0009] The radiation energy for impacting on said polymeric resin is preferably UV, or IR
light, more preferably UV light, and even more preferably UV light with a wavelength
of between 200 nm and 280 nm.
In addition to the radiation activatable resin reactive groups, the fibers can have
a second cross-linking chemical or chemical group, capable to form cross-linking bonds
without being impacted by radiation energy, this second cross-linker group being preferably
selected from the group consisting of aldehyde and urea-based formaldehyde; carboxylic
acid, preferably C2-C9 polycarboxylic acids that contain at least three carboxyl groups,
preferably from the group consisting of citric acid, tartaric acid, malic acid, succinic
acid, glutaric acid, citraconic acid, itaconic acid, tartrate monosuccinic acid, maleic
acid, poly(acrylic acid), poly(methacrylic acid), poly(maleic acid), poly(methylvinylether-co-maleate)
copolymer, poly(methylvinyl ether-co-itaconate copolymer, copolymers of acrylic acid,
and copolymers of maleic acid. The cross-linking can be between cellulose molecules
of the same or of different of the cellulosic fibers.
The present invention also relates to a fibrous aggregate, such as a web, comprising
fibers as described in the above, and this web can have essentially uniform or different,
optionally patterned, degree of cross-linking.
The fibers, or the aggregates are particularly useful and beneficial as being used
as a liquid handling material, and more particular as a material for acquisition and/or
distribution in absorbent bodies, such as disposable absorbent articles.
The present invention further relates to a method for treating cellulosic fibers,
having the steps of a) providing cellulosic fibers; b) forming fiber aggregates; f)
application of radiation activatable resin to the fibers; g) radiation activated curing
of the resin, whereby said steps are executed in the order of b after a). In addition
to these essential steps, the method can further have the optional steps of (d) intermediate
web forming and (e) disintegration; or (h) application of non-radiation activatable
resin, and (i) non-radiation activated cross-linking thereof; also a transporting
step (c) of the fibers or the aggregates can be included. One or more of the process
steps may be repeated. The radiation activatable resin can be selectively applied
to a predetermined region of the formed fiber aggregate, or is selectively applied
to predetermined regions of the formed fiber aggregates at predetermined varying levels.
Detailed description
[0010] Cellulosic fibers of diverse natural origin are applicable to the invention. Although
available from various sources such as Esparto grass, bagasse, kemp, flax, and other
lignaceous and cellulosic fiber containing sources, preferred cellulosic fibers are
derived from wood pulp, especially digested fibers from softwood, hardwood or cotton
linters. Suitable woodpulp fibers for use with the invention can be obtained from
well-known chemical processes such as the Kraft and sulfite processes, with or without
subsequent bleaching. The pulp fibers may also be processed by thermomechanical, chemi-thermo-mechanical
methods, or combinations thereof. The preferred pulp fiber is produced by chemical
methods. Ground wood fibers, recycled or secondary wood pulp fibers, and bleached
and unbleached wood pulp fibers can be used. The preferred starting material is prepared
from long fiber coniferous wood species, such as southern pine, Douglas fir, spruce,
and hemlock. Details of the production of wood pulp fibers are well-known to those
skilled in the art. These fibers are commercially available from a number of companies,
such as from Weyerhaeuser Company, Washington, US, under the designations CF416, NF405,
PL416, FR516, or NB416.
[0011] The fibers may be supplied in slurry, unsheeted or sheeted form. Fibers supplied
as wet lap, dry lap or other sheeted form can be rendered into unsheeted form by mechanically
disintegrating the sheet. The fibers can be provided in a wet or moistened condition,
or can be never-dried fibers. In the case of dry lap, the fibers can be moistened
prior to mechanical disintegration in order to minimize damage to the fibers.
[0012] The fibers can further be treated such as to provide curl or twist to the fibers,
such as resulting from mechanical defibration, or - as a preferred method - from so-called
"flash drying" as being well known in the art, such as described in US-A-5.549.791
(Herron), or U.S. Pat. No. 3,987,968, said patents being hereby expressly incorporated
by reference into this disclosure.
[0013] The fibers can further be treated with cross-linking agents, which are not radiation
activatable but rather allow cross-linking under conventional conditions such as thermal
treatment. Such cellulose crosslinking agents include crosslinking agents known in
the art such as aldehyde and urea-based formaldehyde addition products. See, for example,
US-A- 3,224,926; US-A- 3,241,533, US-A- 3,932,209; US-A-4,035,147, US-A-3,756,913,
US-A-4,689,118; US-A-4,822,453, US-A- 3,440,135, US-A- 4,935,022, US-A- 4,889,595,
US-A- 3,819,470, US-A- 3,658,613, US-A- 4,853,086, all of which are expressly incorporated
herein by reference in their entirety. Other suitable crosslinking agents include
carboxylic acid crosslinking agents such as polycarboxylic acids. US-A-5,137,537;
5,183,707; and US-A-5,190,563 - all being expressly incorporated herein by reference
describe the use of C2-C9 polycarboxylic acids that contain at least three carboxyl
groups (e.g., citric acid and oxydisuccinic acid) as crosslinking agents. Suitable
urea-based crosslinking agents include substituted ureas such as methylolated ureas,
methylolated cyclic ureas, methylolated lower alkyl cyclic ureas, methylolated dihydroxy
cyclic ureas, dihydroxy cyclic ureas, and lower alkyl substituted cyclic ureas. Suitable
polycarboxylic acid crosslinking agents include citric acid, tartaric acid, malic
acid, succinic acid, glutaric acid, citraconic acid, itaconic acid, tartrate monosuccinic
acid, and maleic acid. Other polycarboxylic acid crosslinking agents include polymeric
polycarboxylic acids such as poly(acrylic acid), poly(methacrylic acid), poly(maleic
acid), poly(methylvinylether-co-maleate) copolymer, poly(methylvinyl ether-co -itaconate
copolymer, copolymers of acrylic acid, and copolymers of maleic acid. The use of polymeric
polycarboxylic acid crosslinking agents such as polyacrylic acid polymers, polymaleic
acid polymers, copolymers of acrylic acid, and copolymers of maleic acid is described
in US-A-5.998.511, which is also incorporated herein by reference. Mixtures or blends
of crosslinking agents can also be used.
[0014] Once applied, the crosslinking agents can be treated in conventional ways to effect
crosslinking. For example, the cross-linking agents can be heated at a temperature
and for a time sufficient to cure the crosslinking agent and to provide a crosslinked
fibrous material. Another method for effecting crosslinking is to treat the fibrous
material treated with the cross-linking agent with a crosslinking catalyst and then
optionally heating the resulting web to cure the crosslinking agent. Another conventional
method for crosslinking a fibrous material or a web that includes fibers involves
adjusting the pH of the web to facilitate the crosslinking reaction.
[0015] Cross-linking chemicals suitable as radiation activatable resins are generally of
a polymeric structure, having a polymeric backbone and radiation activatable sites,
i.e. certain chemical groups become chemically active - and hence reactive - only
upon radiation. The term radiation refers in the general context of the present invention
to any radiation, such as electron-beam radiation, or electromagnetic radiation, especially
UV- or IR radiation. The resin can further comprise other reactive sites suitable
for reacting with the cellulosic molecules of the cellulosic fibers or - for example
- conventional cross-linking.
[0016] Upon radiation the radiation activatable groups form radicals which then can bond
to cellulosic molecules of the cellulosic fibers or to other molcules of the resin
itself, such as of the polymeric backbone, thereby forming a cross-linked polymeric
network.
[0017] After the radiation initiated reaction is terminated, there will generally be some
of the network structure with some unreacted sites, respectively some unreacted radiation
activatable groups. Preferably, the reaction is carried out towards high degrees of
cross-linking, preferably to at least 50%, more preferably to at least 70% and even
more preferably to at least 85% of the radiation activatable groups. Further, it is
preferred that the cross-linking reaction is predominantly done such that it includes
radiation activatable groups, i.e. that there are not too many reactions such as between
molecules of the polymeric backbone or other, non-radiation activatable groups. Preferably,
at least 80% more preferably at least 90 % of the created bonds include radiation
activatable groups, as evaluated by
13C-NMR.
[0018] Radiation activatable groups suitable for the present invention are well known in
the art as free radical-generating photoinitiators. The largest group of such groups
are carbonyl compounds, such as ketones, especially α-aromatic ketones. Examples of
α-aromatic ketone photoinitiators include, by way of illustration only, benzophenones;
xanthones and thioxanthones; α-ketocoumarins; benzils; α-Ikoxydeoxybenzoins; benzil
ketals or α,α-dialkoxydeoxybenzoins; benzoyldialkylphosphonates; acetophenones, such
as α-hydroxycyclohexyl phenyl ketone, α,α-dimethyl-α-hydroxyacetophenone, α,α-dimethyl-α-orpholino-4-methzylthioacetophenone,
α-ethyl-α-benzyl-α-dimethylaminoacetophenone, α-ethyl-α-benzyl-α-dimethylamino-4-morpholinoacetophenone,
α-ethyl-α-benzyl-α-dimethylamino-3,4-dimethoxyacetophenone, α-ethyl-α-benzyl-α-dimethylamino-4-methoxyacetophenone,
α-ethyl-α-benzyl-α-dimethylamino-4-imethylaminoacetophenone, α-ethyl-α-benzyl-α-dimethylamino-4-methylacetophenone,
α-ethyl-α-(2-propenyl)-α-dimethylamino-4-morpholinoacetophenone, α,α-bis(2-propenyl)-α-dimethylamino-4-morpholinoacetophenone,α-methyl-α-benzyl-α-dimethylamino-4-orpholinoacetophenone,
and α-methyl-α-(2-propenyl)-α-dimethylamino-4-morpholinoacetophenone; α,α-dialkoxyacetophenones;
α-hydroxyalkylphenones; O-acyl α-oximino ketones; acylphosphine oxides; fluorenones,
such as fluorenone, 2-t-butylperoxycarbonyl-9-fluorenone, 4-t-butylperoxycarbonyl-nitro-9-fluorenone,
and 2,7-di-t-butylperoxycarbonyl-9-fluorenone; and α- and α-naphthyl carbonyl compounds.
Other free radical-generating photoinitiators include, by way of illustration, triarylsilyl
peroxides, such as triarylsilyl t-butyl peroxides; acylsilanes; and some organometallic
compounds. The free radical-generating initiator desirably will be selected from the
group consisting of acetophenones, 4,4'-bis(N, N' - dimethylamino)benzophenones, for
which for safety aspects the residual unreacted level should be minimized or excluded
from human contact, 9,10 (phen)-anthraquinone, for which for safety aspects the residual
unreacted level should be well controlled, benzile, ((2-chloro-)thio-)xanthones, and
even more preferably benzophenones.
[0019] Suitable backbone polymers can be made from a wide variety of monomers such as ethylene;
propylene; vinyl chloride; isobutylene; styrene; isoprene; acrylonitrile; acrylic
acid; methacylic acid; ethyl acrylate; methylmethacrylate; vinyl acrylate; allyl methacrylate;
tripropylene glycol diacrylate; trimethylol propane ethoxylateacrylate; epoxy acrylates,
such as the reaction product of a bisphenol A epoxide with acrylic acid; polyester
acrylates, such as the reaction product of acrylic acid with an adipic acid/hexanediol-based
polyester; urethane acrylates, such as the reaction product of hydroxypropyl acrylate
with diphenylmethane-4,4'-diisocyanate: and polybbutadiene diacrylate oligomer.
[0020] Preferred backbone materials exhibit after polymerization are selected to provide
a T
g of more than 20°C, preferably of more than 30°C, and even more preferably of more
than 50°C.
[0021] Radiation activatable groups and backbones can be combined for example to form (meth)acrylate
copolymers and monoethylenically unsaturated aromatic ketones, which are crosslinkable
by ultraviolet light such as described for the use in contact adhesives, such as has
been described in more detail in U.S.-A- 4,737,559, which is incorporated herein by
reference. Other materials which are crosslinkable by ultraviolet radiation under
atmospheric oxygen and are based on (meth)acrylate copolymers and contain copolymerizable
benzophenone derivatives or acetophenone derivatives, are further detailed in US-A-5.026.806,
which is also incorporated herein by reference. This chemistry has further advantages
over the one as described in U.S.-A- 4,737,559 as it can be crosslinked in the air
(rather than under inert atmosphere), and allows application which is free of solvents
and unsaturated monomers.
[0022] Further suitable radiation activatable resin is an acrylic copolymer combined with
a chemically built-in photoinitiator of the benzophenone type, such as commercially
available for self-adhesive applications from BASF AG, Ludwigshafen, Germany, under
the designation acResin®, adjusted to exhibit a T
g of at least about 30°C. For particular applications, such polymers may include hydrophilizing
agents, such as hydrophilic groups grafted to the polymeric backbone, or so called
surfactants applied to the surface.
[0023] Whilst the resins useful for the present invention can be activated by any kind of
radiation, such as electron beams or infra-red light, preferred executions can be
activated by UV light. More preferably, the exhibit an activation profile as a function
of the wavelength of the radiation, so as to allow better control of the reaction
process, and to minimize pre- and/or post-curing such as through ambient (e.g. solar)
radiation.
[0024] Preferably, the activation activatable resin are neither oxygen activated nor oxygen
inhibited, so as to allow easier operation without the need for particular inert atmosphere.
It is also preferred, that the activatable resins do not exhibit a risk for the safety
of workmen, user, and environment.
[0025] The radiation curable resins are preferably compatible with other additives and /
or applications aids, and non-reactive therewith. The resins can be soluble in solvents,
and are preferably soluble or suspendable in aqueous liquids.
[0026] For the discussion within the present context, the terms "curing" and "cross-linking",
or "curable" and "cross-linkable" can generally be used interchangeably, and refer
to a chemical reaction bonding two active sites of two molecules to each other. In
the present context, the thusly connected molecules are generally polymeric molecules.
This refers to the fact, that the reaction is generally not taking place between the
monomers or oligomers of the "backbone" of the resins as discussed hereinafter, but
that the cross-linking reaction occurs predominantly between already formed polymeric
chains, thusly creating a polymeric network rather than creating polymers by the radiation
activated polymerization.
[0027] As the curing reaction should be predominantly activated by the radiation and not
by thermal effects, the radiation activated reactions should be completed to a sufficient
degree, before the radiation impact increases the temperature so as to also induce
thermally triggered, conventional cross-linking reactions.
[0028] In many applications, it is particularly preferred, that the cured and reacted resins
are stable to further radiation or other reactions conditions such as temperature
and/or pressure and/or hydrolysis conditions. Further, for many applications it will
be desired, that the reacted resins do not exhibit residual stickiness or tack. Similar
to the radiation activatable resins, the reacted resins do not exhibit a risk for
the safety of workmen, user, and environment.
[0029] A process for the treatment of cellulose fibers by radiation curable resins according
to the present invention will include certain process features, as such known in the
art.
a) Providing cellulosic fibers
It is well known for a skilled person, how to provide cellulosic fibers as described
in the above. Thus the fibers can be delivered in an individualized form (i.e. the
fibers are essentially suspended in a carrier means such as gas or a liquid, and do
not form an aggregate as described hereinafter) in the context of the cellulose fiber
production plant, such as in the form of an aqueous slurry, or in an gas suspended
form, such as for pneumatically transported fibers, or a fluidized bed.
Further, the fibers can be treated fibers such as described in the above, such as
by having an increased degree of twist and curl, and/or by comprising a conventional
(i.e. non-radiation activatable) cross-linking resin, optionally in a reacted or unreacted
condition.
b) Forming fiber aggregates
It will also be well know to skilled person that cellulosic fibers can be formed into
a fiber aggregate structure by various means or processes. As used herein, the term
"fiber aggregates" refers to a structure comprising fibers, which are in contact with
each other so as to form this structure. The contact between adjacent fibers can be
established such as based on mechanical effects, such as friction or entanglement,
or chemical effects, such as hydrogen bonding, or cross-linking (which would be referred
to as "inter-fiber cross-linking", which can be achieved by conventional ways of cross-linking,
as discussed separately hereinafter, or by radiation curable cross-linking according
to the present invention) or the like. The contact can also be established such as
by a binding means, such as adhesives, binder resins, or the like. The result of such
aggregation is often referred to as a webs, or sheets, or bales, and the process steps
to form such aggregates in general are known to a skilled person.
Such aggregates can have a wide range of shapes, forms, densities or thickness. For
a preferred application in the field of absorbent articles, the aggregates will preferably
have basis weights of less than about 800 g/m2 and densities of less than about 0.60 g/cm3. Other applications contemplated for the fibers of the present invention include
low density webs having densities which may be less than about 0.03 g/cm3.
Cellulosic fiber aggregates can be further processed to be directly combined with
other elements to form articles - such as absorbent articles.
d) Cellulosic fiber aggregates can also be formed into intermediate structures, such
as roles, spools, or boxed or baled structures, which allow easier interim storage
and/or transport, so as to allow use of such aggregates at different sites than the
manufacturing sites of the aggregates. A particular example is the forming of wet
laid rolls of cellulosic material, which then can be shipped to a "converter site"
where absorbent articles are manufactured comprising the cellulosic material. During
this manufacturing, that aggregates may remain in their original structure and are
inserted into the article upon cutting.
e) The aggregates may also be further disintegrated, such as by well known means such
as hammer-mills, or bale openers, or re-slurrying and so on. Thereafter, a further
aggregate forming step as discussed in the above will be used to form the final aggregate,
now often in a web form.
f) Radiation activatable resin application
In addition to process steps well known as such, radiation activatable resins as discussed
in the above are applied to the cellulose fibers. To this effect, the cellulosic fibers
need to be brought in contact with the respective radiation activatable resins. Whilst
certain forms of application may provide particular benefits for certain or certain
types of radiation activatable resins, a particular form of application has not been
found to be critical for the present invention.
This contacting can be achieved while the cellulosic fibers are individualized or
while these fibers are in an aggregate form, such as a web. If the fibers are in a
defibrated state, they can be in a low density, individualized, fibrous form known
as "fluff", as discussed in the above.
The resin may be applied to the fibers by means of a carrier or solvent liquid, such
as an aqueous solution or suspension comprising the resin. The carrier liquid and
the resin can then be contacted with the fiber by generally known methods, including
forming an aqueous slurry of the fibers and adding the resin, optionally by the means
of the carrier, to the slurry. Upon dewatering the slurry, the resin can deposit on
the fibers or actually penetrate into the fiber. The resin may be applied to the fibers
while these are in an essentially individualized state, such as by being suspended
in an air stream, such as by spraying the resin with or without a carrier. The fibers
may be formed into a structure, such as bales or sheets, and the prior to treatment
with the reactive agents, following methods as described hereinafter in the context
of forming webs.
As used herein, "effective amount" refers to an amount of agent sufficient to provide
an improvement in at least one significant absorbency property of the fibers themselves
and/or absorbent structures containing the crosslinked fibers, relative to uncrosslinked
fibers. As will be readily apparent to a skilled person, the amount of the agent will
depend on chemical composition with regard to the amount of radiation activatable
groups relative to the backbone polymer. Amounts of 20% on a weight basis relative
to the amount of fibers and resin (and thus excluding a carrier, if used) are not
untypical, although not only for economical reasons smaller amounts such as less than
about 15% are preferred, whilst often more than about 0.5 %, preferably more than
1% and often more than 5% will be required so as to provide sufficient degree of cross-linking.
g) Radiation activated cross-linking
After the radiation activatable resins have been applied to the cellulose fibers,
the resins need to be submitted to a radiation, which is suitable to activate the
cross-linking reaction as described herein before for the resins as such.
[0030] The radiation useful to activate the cross-linking reaction is depending on the particular
chemistry of the reagents, and may be electromagnetic (including visual light, UV-A,
B, C, or IR), or electron-beams as has been discussed in the above.
[0031] A preferred execution is the use of UV-light, and even more preferred is the use
of UV-C light, such as having a wavelength of from about 200 nm to about 280 nm, in
particular, when the radiation activatable groups are benzophenone groups. However,
also UV-A light in the range of 315nm to 400nm can be used advantageously. A particular
benefit of using such wavelengths lies in readily available equipment (i.e. mercury-vapor
lamps such as commercially available from IST Metz GmbH, Nuertingen, Germany, such
as providing between 160 W/cm of length of lamp and 200 W/cm, using mercury vapor
as being particularly suitable for UV-C sensitive reagents, or using iron doped metal
halides for UV-A/B sensitive materials) as well as in insensitivity to visual / sun-light,
such that no particular precautions with regard to preventing of undesired reaction
need to be taken during or after radiating for the reaction.
[0032] The energy level required to perform the respective reaction is depending on the
particular chemistry, on the degree of desired cross-linking, and on the amount of
material treated per time and/or area unit. Further, it depends on the relative positioning
of the fibers and the radiation emitting element, i.e. the lamps. Generally, it has
been found, that the intensity is highly important for executing the reaction, such
that by applying high radiation intensities for short periods good reaction completeness
can be achieved without straining other material properties, such as color, by high
energy input.
[0033] There can be various relative positions between the fibers which are to be radiated,
and the radiation emitting source (e.g. lamps). For example, if the fibers are positioned
in a layered (web) arrangement, there will be a certain penetration of the radiation
into the web, which can be used for a desired degree of cross-linking. If this would
not be desired, other arrangements can be chosen, such as having fibers moving freely
in a radiated duct. The apparatus may further comprise mirrors to distribute the radiation
more evenly or to focus the radiation to certain regions.
[0034] In one embodiment, the crosslinking agent is caused to react with the fibers in the
substantial absence of interfiber bonds, i.e., while interfiber contact is maintained
at a low degree of occurrence relative to unfluffed pulp fibers, or the fibers are
submerged in a solution that does not facilitate the formation of interfiber bonding,
especially hydrogen bonding. Alternatively, if desired, the cross-linking can be used
to create inter-fiber crosslinking.
[0035] Apart from optional repetition of any of the described process steps, further steps
can be added, which might provide further benefits to the materials, products, or
processes.
[0036] In particular, when it is desired to not only have radiation activated cross-linking,
conventional cross-linking can be included by
h) application of a crosslinking agent to the fibers, which is not radiation curable,
and
i) submitting the such treated fibers to cross-linking conditions without the application
of radiation, such as thermal treatment.
[0037] Further, when forming the fibrous material, a further process step can be
k) the addition of other materials to the cellulosic fibers, such as synthetic fibers,
or particulate materials, such as powders or granules. In order to still maintain
the predominantly cellulosic fiber dominated properties of the fibrous material, the
ammount of the added material should not be excessive, and typically will not exceed
50% of the total fibrous material.
Added synthetic fibers can be made from a variety of polymers, including thermoplastic
polyolefins such as polyethylene (e.g., PULPEX®) and polypropylene, polyesters, copolyesters,
polyvinyl acetate, polyethylvinyl acetate,polyvinyl chloride, polyvinylidene chloride,
polyacrylics, polyamides, copolyamides, polystyrenes, polyurethanes and the like.
Suitable fibers may also be made from superabsorbent material, such as well known
in the art. Depending on the particular intended application, suitable fibrous materials
may include hydrophobic fibers that have been made hydrophilic, such as by incorporating
hydrophilizing agents into the resin, or by treating the surface. Suitable thermoplastic
fibers can be made from a single polymer (monocomponent fibers), or can be madefrom
more than one polymer (e.g., bicomponent fibers such as sheath/core fibers). The length
of the synthetic fibers can vary over a wide range, and typically, these thermoplastic
fibers have a length from about 0.3 to about 7.5 cm, preferably from about 0.4 to
about 3.0 cm, and most preferably from about 0.6 to about 1.2 cm. The diameter of
these thermoplastic fibers is typically defined in terms of either denier (grams per
9000 meters) or decitex (grams per 10,000 meters). Suitable thermoplastic fibers can
have a decitex in the range from about 1.0 to about 20, preferably from about 1.4
to about 10, and most preferably from about 1.7 to about 3.3.
The fibrous material may further comprise particulate material, which may be added
for enhancing the strength properties of the web, and can be polymeric particles,
optionally partially molten so as to provide a binder function. Such particles may
be added for enhancing fluid handling properties, such as when using so called superabsorbent
materials, or may be added for improving gas or odor adsorption properties. Thus suitable
particles may be made of partially cross-linked polyacrylate, or silica, or zeolithes
or any other natural or synthetic material. The individual particle size is typically
not larger than about 1000 µm, and it will often be desired for handling and dust
related reasons limited amounts of particles smaller than about 50 µm.
[0038] A particular aspect of the present invention relates to the order of the various
process steps. In the above, the following process steps were identified:
a) providing cellulosic fibers;
b) forming fiber aggregates;
c) transporting;
d) intermediate web forming;
e) intermediate disintegration
f) application of radiation activatable resin;
g) radiation activated curing;
h) application of non-radiation activatable resin;
i) non-radiation activated cross-linking;
k) addition of other materials.
Out of these steps, a), b), f) and g) are considered to be essential process steps
for executing the present invention - the remaining steps c), d), e), h), i) and k)
as well as repetition of certain steps are considered optional. Whilst the above steps
are not listed in the order as these could be executed in the treatment process, certain
of these steps have a certain relative order. Especially, the radiation activatable
resin application (step f) needs to be executed before the radiation takes place (step
g), and the same principle applies to the non-radiation activated resin application
(step h) before the non-radiation curing (step i). Also, an interim disintegration
step (e) can only be executed after an aggregate has been formed - (either step b)
or d)).
[0039] The process according to the present invention can be executed by applying the radiation
curable resin to the fibers at any stage of the fiber handling process, and it also
allows the radiation activation to be executed at any stage thereafter.
[0040] For example, conventional fluff pulp fibers can be treated with the radiation activatable
resin, either in a fluffed state or when being formed into an aggregate or a web,
and can be radiation cured whilst the individualized fluff is further conveyed through
an activation pipe, wherein it is radiated.
[0041] Consequently, in such a process, the cross-linking can be applied evenly to all individualized
fibers. The radiation can also be applied to a web formed from fibers to which the
radiation activatable resin has been applied, or to a web formed from fibers without
resin being added to the fibers, but added to the web as such. In either case, the
radiation can be applied to the web such that a homogeneous cross-linking is achieved.
This can be performed by controlling the thickness of the web such that the radiation
can penetrate sufficiently into the web. The radiation can also be applied predetermined
and selectively to the web, such that particular property profile can be designed
into the web, such as by creating a cross-linking profile through the thickness dimension
of a web. Considering a web which is to be introduced into an absorbent article, there
could be a higher degree of cross-linking either by application of more radiation
activatable resin or by more radiation, in the liquid loading receiving region, thereby
imparting better gush handling properties, whilst other regions of the web further
remote form the loading receiving region have better liquid retention properties by
a lesser degree of cross-linking.
[0042] In one particular embodiment, the present invention relates to a process including
the transport (step c) of fibers from one location to another, such as from the pulp
mill to an article production site. In the context of this discussion, "transport"
refers to an operation where the fibers are in an aggregate form allowing non-continuous
transport such as in rolls or bales or bags, but also an interim storage. Thus, the
direct transport such as in continuous piping system within one production site or
between subsites of on site would be excluded under this term, but the conveying into
a interim storage bin decoupling the fiber delivery from the fiber removal from that
bin, would be included under the term transport with interim storage.
[0043] Considering conventional cross-linking technologies, such as discussed in the background
section, the cross-linking is formed at the pulp producing site, and the cross-linked
fibers are transported to the converter site for further processing, such as forming
articles, such as absorbent articles. However, as the cross-linking step aims at modifying
the properties of the fibers, these properties can be partially lost during the transport.
Such as for the use in absorbent articles, it is often desired that the cross-linking
step improves the liquid handling properties of the fibers and the webs made thereof,
or articles comprising such fibers, and often this improvement is achieved by modifying
the fibers towards better wet and dry resiliency or stiffness under a load. Also,
in order to get a more open structure, the fibers have been modified towards higher
bulk, such as by imparting twist and curl to the cellulosic fibers.
[0044] These effects, however, imply a more difficult handling for transport of the fibers
and/or the risk of fiber damage during this transport, such that the fibers may loose
some of the benefits as imparted by the original treatment. Known attempts to address
this problem are low density packaging, such as in a bale form at a lower density
than conventional wet laid roll forming would imply. An further approach ahs been
described in EP-A-0.705.365 (STORA), wherein an alcohol can be added to the fibers
allowing the fibers to be transported between application of the cross-linker resin
and the cross-linking step. However, due to the use of conventional cross-linking
agents requiring thermal treatment to activate the cross-linking, the process after
the transporting step requires significant effort from an equipment point of view.
Also, the addition of the alcohols can impact on the properties of the fibers and/or
of the resulting webs or products.
[0045] For such circumstances, the present invention allows alternative process configurations
by better and easier application of the cross-linker resins more independently from
the curing step.
[0046] The step of the addition of further additives (step k), such as other fibers, or
particles, can be introduced into the process at many points, depending on the type
of materials, including the combination with the cellulosic fibers before the radiation
activatable resin is added, or thereafter. If the resin is applied to the fibrous
material after the non-cellulosic material has been added, the resin may also react
with parts of the added material, or may react on the surface of such materials.
[0047] A preferred process for treating cellulose fibers comprises the steps of (in terms
of the reference to the process step list in the above) in the following order:
a) providing the cellulosic fiber at the fiber production site, such as a pulp mill;
f) application of radiation activatable resin to the fibers at the same site;
d) forming a fiber aggregate at the fiber production site, such as in roll for, or
bale form;
c) transporting the aggregate to an article manufacturing plant, such as a diaper
plant;
e) disintegrating the fibers;
g) curing the fibers by ration treatment;
b) forming the final web and combining it to an article.
[0048] In a modification of this process, the step g) of curing the fibers, can also be
executed after the final web has been formed (step b).
[0049] A further preferred process option would additionally include conventional crosslinking
at the pulp mill production site, such that the order of process steps would be as
follows:
a) providing the cellulosic fiber at the pulp mill production site;
f) application of radiation activatable resin to the fibers at the same site;
h) application of non-radiation activatable resin to the fibers;
i) heat treating the fibers so as to cure the non-radiation activatable resin;
d) forming a fiber aggregate at the fiber production site such as in roll for, or
bale form;
c) transporting the aggregate to a converter plant, such as a diaper plant;
e) disintegrating the fibers;
g) curing the fibers by ration treatment;
b) forming the final web and combining it to an article.
[0050] In analogy to the above, the step g) of curing the fibers, can also be executed after
the final web has been formed (step b). Also, the application of the radiation activatable
resin and the non-radiation activatable resin can be done simultaneously in one step,
and it can e achieved by adding one resin including radiation activatable groups as
well as conventional cross-linking groups.
[0051] Yet another preferred process option would comprise the following order of process
steps:
a) providing the cellulosic fiber at the pulp mill production site;
f1) application of a first radiation activatable resin to the fibers at the same site;
g1) curing the fibers by a first ration treatment;
d) forming a fiber aggregate at the pulp mill, such as in roll or bale form;
c) transporting the aggregate to a converter plant, such as a diaper plant;
e) disintegrating the fibers;
f2) application of a second radiation activatable resin to the fibers;
g2) curing the fibers by second ration treatment;
b) forming the final web and combining it to an article.
[0052] Yet a further preferred process option includes conventional crosslinking at the
pulp mill production site, and both radiation activatable resin application and curing
at the converter side, such that the order of process steps would be as follows:
a) providing the cellulosic fiber at the pulp mill production site;
h) application of non-radiation activatable resin to the fibers;
i) heat treating the fibers so as to cure the non-radiation activatable resin;
d) forming a fiber aggregate at the pulp mill, such as in roll for, or bale form;
c) transporting the aggregate to a converter plant, such as a diaper plant;
e) disintegrating the fibers;
f) application of radiation activatable resin to the fibers at the same site;
g) curing the fibers by ration treatment;
b) forming the final web and combining it to an article.
[0053] In analogy to the above, the step g) of curing the fibers, can also be executed after
the final web has been formed (step b).
[0054] As also done in conventional processes, an additional drying step, and preferably
a flash drying step, can be performed between steps h) (non-radiation activatable
resin application) and i) (no-radiation activatable curing). This step could provide
increased twist and curl of the fibers so as to improve liquid handling functionality.
[0055] Fibers according to the present invention exhibit beneficial performance properties,
such as when evaluated upon being formed into a web suitable for testing, for example
at densities and basis weights suitable for use in articles such as absorbent articles.
[0056] In particular, such webs can be evaluated according to the Capillary sorption test
as described explicitly in WO 99/45879 in the test method section, which is incorporated
herein by reference, and preferably exhibit "Capillary Sorption Desorption Height
at which the material has released 50% of its capacity at 0 cm (i.e. of CSAC 0), (CSDH
50), sometimes also referred to as "Medium Desorption Pressure" expressed in cm, of
less than 20 cm, more preferably of less than 17cm and even more preferably of less
than 15 cm.
[0057] Such webs preferably have an overall uptake value, as measured by the same test method
as the Capillary Sorption Absorbent Capacity at a height of 0 cm (CSAC 0) expressed
in units of g {of fluid} / g {of material} of more than 10g/g, more preferably of
more than 12 g/g and even more preferably of more than 14g/g.
[0058] It further has been found, that fibers according to the present invention exhibit
a lower loss of brightness during cross-linking, as compared to fibers cross-linked
by conventional cross-linking methods, as described before. In particular, when using
ISO Standards 2469 "Paper, board, and pulps - Measurement of diffuse reflectance factor,"
2470 "Paper and Board - Measurement of Diffuse Blue Reflectance Factor (ISO Brightness)"
and 3688 "Pulps -- Measurement of Diffuse Blue Reflectance Factor (ISO Brightness)",
it has been found that for fibers cross-linked by methods according to the present
invention a loss in brightness is less than about 7%, and preferably less than about
3 %, such as by reducing the brightness of untreated fiber of 83% to values higher
than 75%, preferably higher than 80%, whilst comparable conventional cross-linking
conditions can result in brightness losses of more than 7%, i.e. to less than 76 %
for the same example.
[0059] Whilst fibers treated according to the present invention can be used for a broad
field of applications, such as filtration fiber, fillings, insulation and the like,
a preferred use is for liquid handling materials, and especially for absorbent articles,
such as disposable diapers for babies and/or adults, feminine care articles, such
as so called catamenial pads or tampons, and the like.
[0060] It has been found that the cross-linked fibers of the present invention can be used
to make absorbent cores having substantially improved fluid handling properties including,
but not limited to, liquid acquisition rate, liquid distribution rate, and interim
liquid storage capacity relative to equivalent density absorbent cores made from conventional
uncross-linked fibers or from prior known cross-linked fibers. Furthermore, these
improved absorbency results may be obtained in conjunction with increased levels of
wet resiliency. The term wet resilience, in the present context, refers to the ability
of a moistened pad to spring back towards its original shape and volume upon exposure
to and release from compression forces. Compared to cores made from untreated cellulosic
fibers, and prior known cross-linked fibers, the absorbent cores made from the fibers
of the present invention will regain a substantially higher proportion of their original
volumes upon release of wet and dry compression forces.
1. Fibrous material comprising cellulosic fibers, characterized in that
said fibers comprise a polymeric resin comprising covalently bonded radiation reactive
groups, which is capable of forming cross-linking bonds upon being impacted by radiation
energy.
2. Fibrous material according to claim 1, wherein said cellulose based fibers are crimped,
curled, preferably flash-dried fibers.
3. Fibrous material according to claim 1 or 2, wherein said polymeric resin has a Tg of more than 30°C, preferably 50°C, when cross-linked to a degree of cross-linking
of at least 85%.
4. Fibrous material according to claim 3, wherein said radiation activatable groups are
selected from the group consisting of benzophenone, anthraquinone, benzile, xanthones,
preferably from the group of benzophenones.
5. Fibrous material according to claim 3.3, wherein said resin comprises a polymeric
backbone comprising monomer molecules selected from the group of ethylene; propylene;
vinyl chloride; isobutylene; styrene; isoprene; acrylonitrile; acrylic acid; methacylic
acid; ethyl acrylate; methylmethacrylate; vinyl acrylate; allyl methacrylate; tripropylene
glycol diacrylate; trimethylol propane ethoxylateacrylate; epoxy acrylates; polyester
acrylates; and urethane acrylates.
6. Fibrous material according to any of the preceding claims, wherein said polymeric
resin is applied in amounts of less than 50% by weight of fibers and resin in the
unreacted state, preferably in amounts of less than 25% and even more preferably in
amounts of less than 15 %.
7. Fibrous material according to any of the preceding claims, wherein said polymeric
resin is applied in amounts of more than 0.25% in its reacted state, preferably more
than 1%, and more preferably more than 5%.
8. Fibrous material according to any of the preceding claims, wherein said polymeric
resin is dissolvable or dispersible in a liquid carrier, said carrier preferably being
water.
9. Fibrous material according to any of the preceding claims wherein said radiation energy
for impacting on said polymeric resin is selected from the group of UV, IR light,
preferably UV light, more preferably is UV light with a wavelength of between 200
nm and 280 nm.
10. Fibrous material according to any of the preceding claims, further comprising a second
cross-linking material capable to form cross-linking bonds without being impacted
by radiation energy.
11. Fibrous material according to claim 10, wherein said second crosslinker is selected
from the group consisting of aldehyde and urea-based formaldehyde; carboxylic acid,
preferably C2-C9 polycarboxylic acids that contain at least three carboxyl groups,
preferably from the group consisting of citric acid, tartaric acid, malic acid, succinic
acid, glutaric acid, citraconic acid, itaconic acid, tartrate monosuccinic acid, maleic
acid, poly(acrylic acid), poly(methacrylic acid), poly(maleic acid), poly(methylvinylether-comaleate)
copolymer, poly(methylvinyl ether-co -itaconate copolymer, copolymers of acrylic acid,
and copolymers of maleic acid.
12. Fibrous material according to any of the preceding claims wherein said cross-linking
is cross-linking between cellulose molecules of the same or of different of said cellulosic
fibers.
13. A fibrous aggregate comprising fibers according to any of the preceding claims.
14. A fibrous aggregate according to claim 13 comprising at least two preselected regions
of different degree of cross-linked radiation activatable polymeric resin.
15. A fibrous aggregate according to claim 14, wherein said at least two preselected regions
have a different relative amount of said polymeric resin applied thereto.
16. A liquid handling material for use in an absorbent body comprising fibers according
to any of claims 1 to 12, or a fibrous aggregate according to any of claims 13 - 15.
17. A liquid handling material according to claim 16 for use as an acquisition distribution
material in an absorbent body.
18. Method for treating cellulosic fibers, said method comprising the steps of
a) providing cellulosic fibers;
b) forming fiber aggregates;
characterized in that
it further comprises the steps of
f) application of radiation activatable resin to said fibers;
g) radiation activated curing of said resin;
whereby said steps are executed in the order of b after a).
19. Method according to claim 18, further comprising the process steps of intermediate
web forming (d) and disintegration (e); or application of non-radiation activatable
resin (h) and non-radiation activated cross-linking thereof (i); or transporting (c)
said fibers or said web.
20. Method according to claim 18 or 19, wherein one or more of the steps application of
radiation activatable resin to said fibers (f); or radiation activated curing of said
resin (g); or intermediate web forming (d) and disintegration (e); or application
of non-radiation activatable resin (h) and non-radiation activated cross-linking thereof
(i); or transporting (c) said fibers or said web is executed more than one time.
21. A method according to any of claims 18 - 20, wherein said radiation activatable resin
is selectively applied to a predetermined region of the formed fiber aggregate, or
is selectively applied to predetermined regions of the formed fiber aggregates at
predetermined varying levels.
22. A method according to any of claims 18 - 21, wherein said radiation activatable resin
curing radiation is applied to predetermined regions of said formed fiber aggregate.
23. A method according to any of claims 18 - 22, wherein said radiation activatable resin
is activated by being exposed to UV radiation, preferably of the wavelength between
200 nm and 280 nm.
24. A method according to any of claims 18 - 23, wherein said radiation activatable resin
is applied at preselected varying intensity for preselected different regions of said
aggregates.